Fix crash in CheckNoMutatedEs6Exports on 'export default <expression>'

Fixes #3453

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=263617675

src/com/google/javascript/jscomp/lint/CheckNoMutatedEs6Exports.java

@@ -111,6 +111,18 @@ private void visitExport(Node export) {
           checkState(lhs.isName());
           exportedLocalNames.add(lhs.getString());
         }
+      } else if (export.getBooleanProp(Node.EXPORT_DEFAULT)) {
+        // export default function() {}
+        // export default function foo() {}
+        // export default 0;
+        // Note: default exports may or may not be declarations.
+        if (!declaration.isClass() && !declaration.isFunction()) {
+          return;
+        }
+        Node nameNode = declaration.getFirstChild();
+        if (!nameNode.isEmpty() && !nameNode.getString().isEmpty()) {
+          exportedLocalNames.add(nameNode.getString());
+        }
       } else {
         // export function foo() {}
         // export class Bar {}

test/com/google/javascript/jscomp/lint/CheckNoMutatedEs6ExportsTest.java

@@ -55,7 +55,16 @@ public void testMutatedInInnerScopeIsError() {
     testWarning("let x = 0; export {x as y}; () => x++;", MUTATED_EXPORT);
     testWarning("export function foo() {}; () => foo = 0;", MUTATED_EXPORT);
     testWarning("export default function foo() {}; () => foo = 0;", MUTATED_EXPORT);
+    testWarning("export default class Foo {}; () => Foo = 0;", MUTATED_EXPORT);
     testWarning("export function foo() { foo = 0; };", MUTATED_EXPORT);
     testWarning("export let x = 0; export function foo() { x++; };", MUTATED_EXPORT);
   }
+
+  @Test
+  public void testExportDefaultExpressionIsOk() {
+    testSame("export default 0;");
+    testSame("export default {prop: 0}");
+    testSame("export default function() {};");
+    testSame("export default class {};");
+  }
 }