Add constructor functions for {Symbol,Node}Links

Using a constructor function like this can help node better optimize
object allocation. This improves memory usage when compiling
`src/compiler` from **277M** to **270M**, a nice ~3% win.

src/compiler/checker.ts

@@ -1252,15 +1252,22 @@ namespace ts {
             }
         }
 
+        function SymbolLink(this: SymbolLinks) {
+        }
+
         function getSymbolLinks(symbol: Symbol): SymbolLinks {
             if (symbol.flags & SymbolFlags.Transient) return <TransientSymbol>symbol;
             const id = getSymbolId(symbol);
-            return symbolLinks[id] || (symbolLinks[id] = {});
+            return symbolLinks[id] || (symbolLinks[id] = new (<any>SymbolLink)());
+        }
+
+        function NodeLink(this: NodeLinks) {
+            this.flags = 0;
         }
 
         function getNodeLinks(node: Node): NodeLinks {
             const nodeId = getNodeId(node);
-            return nodeLinks[nodeId] || (nodeLinks[nodeId] = { flags: 0 } as NodeLinks);
+            return nodeLinks[nodeId] || (nodeLinks[nodeId] = new (<any>NodeLink)());
         }
 
         function isGlobalSourceFile(node: Node) {