Revert "OK, time to see if Map is the perf issue"

This reverts commit 73cb712.

Author: weswigham

src/compiler/checker.ts

@@ -563,7 +563,7 @@ namespace ts {
         const flowLoopNodes: FlowNode[] = [];
         const flowLoopKeys: string[] = [];
         const flowLoopTypes: Type[][] = [];
-        let sharedFlowTypes: MapLike<FlowType> = createDictionaryObject();
+        const sharedFlowTypes: Map<FlowType> = createMap();
         const potentialThisCollisions: Node[] = [];
         const potentialNewTargetCollisions: Node[] = [];
         const awaitedTypeStack: number[] = [];
@@ -16063,7 +16063,7 @@ namespace ts {
                         // some getFlowTypeOfReference invocation. A node is considered shared when it is the
                         // antecedent of more than one node.
                         const nodeId = `${refKey}@${getFlowNodeId(flow)}`;
-                        const result = sharedFlowTypes[nodeId];
+                        const result = sharedFlowTypes.get(nodeId);
                         if (result) {
                             flowDepth--;
                             return result;
@@ -16132,7 +16132,7 @@ namespace ts {
                     if (flags & FlowFlags.Shared && !isIncomplete(type)) {
                         // Record visited node and the associated type in the cache.
                         const flowId = `${refKey}@${getFlowNodeId(flow)}`;
-                        sharedFlowTypes[flowId] = type;
+                        sharedFlowTypes.set(flowId, type);
                     }
                     flowDepth--;
                     return type;
@@ -28956,7 +28956,7 @@ namespace ts {
                 if (skipTypeChecking(node, compilerOptions)) {
                     return;
                 }
-                sharedFlowTypes = createDictionaryObject(); // We clear this each time we start checking a file so memory usage doesn't grow too much due to flow result caching - we can't really fully clear at any smaller bounds (just erase subsets, if we wanted to track them)
+                sharedFlowTypes.clear(); // We clear this each time we start checking a file so memory usage doesn't grow too much due to flow result caching - we can't really fully clear at any smaller bounds (just erase subsets, if we wanted to track them)
 
                 // Grammar checking
                 checkGrammarSourceFile(node);

src/compiler/core.ts

@@ -72,7 +72,7 @@ namespace ts {
     export const emptyArray: never[] = [] as never[];
 
     /** Create a MapLike with good performance. */
-    export function createDictionaryObject<T>(): MapLike<T> {
+    function createDictionaryObject<T>(): MapLike<T> {
         const map = Object.create(/*prototype*/ null); // tslint:disable-line:no-null-keyword
 
         // Using 'delete' on an object causes V8 to put the object in dictionary mode.