diff --git a/packages/@ngtools/webpack/src/compiler_host.ts b/packages/@ngtools/webpack/src/compiler_host.ts
index b1ff333a8944..1cccd61887f8 100644
--- a/packages/@ngtools/webpack/src/compiler_host.ts
+++ b/packages/@ngtools/webpack/src/compiler_host.ts
@@ -68,6 +68,10 @@ export class VirtualFileStats extends VirtualStats {
   set content(v: string) {
     this._content = v;
     this._mtime = new Date();
+    this._sourceFile = null;
+  }
+  setSourceFile(sourceFile: ts.SourceFile) {
+    this._sourceFile = sourceFile;
   }
   getSourceFile(languageVersion: ts.ScriptTarget, setParentNodes: boolean) {
     if (!this._sourceFile) {
@@ -96,6 +100,8 @@ export class WebpackCompilerHost implements ts.CompilerHost {
   private _basePath: string;
   private _setParentNodes: boolean;
 
+  private _cache: boolean = false;
+
   constructor(private _options: ts.CompilerOptions, basePath: string) {
     this._setParentNodes = true;
     this._delegate = ts.createCompilerHost(this._options, this._setParentNodes);
@@ -129,6 +135,10 @@ export class WebpackCompilerHost implements ts.CompilerHost {
     this._changed = true;
   }
 
+  enableCaching() {
+    this._cache = true;
+  }
+
   populateWebpackResolver(resolver: any) {
     const fs = resolver.fileSystem;
     if (!this._changed) {
@@ -156,21 +166,33 @@ export class WebpackCompilerHost implements ts.CompilerHost {
     this._changed = false;
   }
 
+  invalidate(fileName: string): void {
+    this._files[fileName] = null;
+  }
+
   fileExists(fileName: string): boolean {
     fileName = this._resolve(fileName);
-    return fileName in this._files || this._delegate.fileExists(fileName);
+    return this._files[fileName] != null || this._delegate.fileExists(fileName);
   }
 
   readFile(fileName: string): string {
     fileName = this._resolve(fileName);
-    return (fileName in this._files)
-         ? this._files[fileName].content
-         : this._delegate.readFile(fileName);
+    if (this._files[fileName] == null) {
+      const result = this._delegate.readFile(fileName);
+      if (result !== undefined && this._cache) {
+        this._setFileContent(fileName, result);
+        return result;
+      } else {
+        return result;
+      }
+    }
+    return this._files[fileName].content;
   }
 
   directoryExists(directoryName: string): boolean {
     directoryName = this._resolve(directoryName);
-    return (directoryName in this._directories) || this._delegate.directoryExists(directoryName);
+    return (this._directories[directoryName] != null)
+        || this._delegate.directoryExists(directoryName);
   }
 
   getFiles(path: string): string[] {
@@ -198,8 +220,11 @@ export class WebpackCompilerHost implements ts.CompilerHost {
   getSourceFile(fileName: string, languageVersion: ts.ScriptTarget, onError?: OnErrorFn) {
     fileName = this._resolve(fileName);
 
-    if (!(fileName in this._files)) {
-      return this._delegate.getSourceFile(fileName, languageVersion, onError);
+    if (this._files[fileName] == null) {
+      const content = this.readFile(fileName);
+      if (!this._cache) {
+        return ts.createSourceFile(fileName, content, languageVersion, this._setParentNodes);
+      }
     }
 
     return this._files[fileName].getSourceFile(languageVersion, this._setParentNodes);
diff --git a/packages/@ngtools/webpack/src/plugin.ts b/packages/@ngtools/webpack/src/plugin.ts
index bb309206cad7..cde52330d928 100644
--- a/packages/@ngtools/webpack/src/plugin.ts
+++ b/packages/@ngtools/webpack/src/plugin.ts
@@ -165,6 +165,10 @@ export class AotPlugin implements Tapable {
     this._program = ts.createProgram(
       this._rootFilePath, this._compilerOptions, this._compilerHost);
 
+    // We enable caching of the filesystem in compilerHost _after_ the program has been created,
+    // because we don't want SourceFile instances to be cached past this point.
+    this._compilerHost.enableCaching();
+
     if (options.entryModule) {
       this._entryModule = options.entryModule;
     } else if ((tsConfig.raw['angularCompilerOptions'] as any)
@@ -194,6 +198,10 @@ export class AotPlugin implements Tapable {
   apply(compiler: any) {
     this._compiler = compiler;
 
+    compiler.plugin('invalid', (fileName: string, timestamp: number) => {
+      this._compilerHost.invalidate(fileName);
+    });
+
     // Add lazy modules to the context module for @angular/core/src/linker
     compiler.plugin('context-module-factory', (cmf: any) => {
       cmf.plugin('after-resolve', (result: any, callback: (err?: any, request?: any) => void) => {
@@ -251,6 +259,7 @@ export class AotPlugin implements Tapable {
     if (this._compilation._ngToolsWebpackPluginInstance) {
       return cb(new Error('An @ngtools/webpack plugin already exist for this compilation.'));
     }
+
     this._compilation._ngToolsWebpackPluginInstance = this;
 
     this._resourceLoader = new WebpackResourceLoader(compilation);
@@ -284,18 +293,20 @@ export class AotPlugin implements Tapable {
           this._rootFilePath, this._compilerOptions, this._compilerHost, this._program);
       })
       .then(() => {
-        const diagnostics = this._program.getGlobalDiagnostics();
-        if (diagnostics.length > 0) {
-          const message = diagnostics
-            .map(diagnostic => {
-              const {line, character} = diagnostic.file.getLineAndCharacterOfPosition(
-                diagnostic.start);
-              const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');
-              return `${diagnostic.file.fileName} (${line + 1},${character + 1}): ${message})`;
-            })
-            .join('\n');
-
-          throw new Error(message);
+        if (this._typeCheck) {
+          const diagnostics = this._program.getGlobalDiagnostics();
+          if (diagnostics.length > 0) {
+            const message = diagnostics
+              .map(diagnostic => {
+                const {line, character} = diagnostic.file.getLineAndCharacterOfPosition(
+                  diagnostic.start);
+                const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, '\n');
+                return `${diagnostic.file.fileName} (${line + 1},${character + 1}): ${message})`;
+              })
+              .join('\n');
+
+            throw new Error(message);
+          }
         }
       })
       .then(() => {
diff --git a/packages/@ngtools/webpack/src/refactor.ts b/packages/@ngtools/webpack/src/refactor.ts
index 105920bb65a2..57a2e6937277 100644
--- a/packages/@ngtools/webpack/src/refactor.ts
+++ b/packages/@ngtools/webpack/src/refactor.ts
@@ -60,12 +60,15 @@ export class TypeScriptFileRefactor {
     if (!this._program) {
       return [];
     }
-    let diagnostics: ts.Diagnostic[] = this._program.getSyntacticDiagnostics(this._sourceFile)
-                              .concat(this._program.getSemanticDiagnostics(this._sourceFile));
+    let diagnostics: ts.Diagnostic[] = [];
     // only concat the declaration diagnostics if the tsconfig config sets it to true.
     if (this._program.getCompilerOptions().declaration == true) {
       diagnostics = diagnostics.concat(this._program.getDeclarationDiagnostics(this._sourceFile));
     }
+    diagnostics = diagnostics.concat(
+      this._program.getSyntacticDiagnostics(this._sourceFile),
+      this._program.getSemanticDiagnostics(this._sourceFile));
+
     return diagnostics;
   }