Allow use before declaration for export= assignments (#17967) (#17972)

* Allow use before declaration for export= assignments (#17967)

* Add release-2.5 to covered branches

Author: weswigham

.travis.yml

@@ -16,6 +16,7 @@ matrix:
 branches:
   only:
   - master
+  - release-2.5
 
 install:
   - npm uninstall typescript --no-save

src/compiler/checker.ts

@@ -791,12 +791,17 @@ namespace ts {
             // 2. inside a function
             // 3. inside an instance property initializer, a reference to a non-instance property
             // 4. inside a static property initializer, a reference to a static method in the same class
+            // 5. inside a TS export= declaration (since we will move the export statement during emit to avoid TDZ)
             // or if usage is in a type context:
             // 1. inside a type query (typeof in type position)
-            if (usage.parent.kind === SyntaxKind.ExportSpecifier) {
+            if (usage.parent.kind === SyntaxKind.ExportSpecifier || (usage.parent.kind === SyntaxKind.ExportAssignment && (usage.parent as ExportAssignment).isExportEquals)) {
                 // export specifiers do not use the variable, they only make it available for use
                 return true;
             }
+            // When resolving symbols for exports, the `usage` location passed in can be the export site directly
+            if (usage.kind === SyntaxKind.ExportAssignment && (usage as ExportAssignment).isExportEquals) {
+                return true;
+            }
 
             const container = getEnclosingBlockScopeContainer(declaration);
             return isInTypeQuery(usage) || isUsedInFunctionOrInstanceProperty(usage, declaration, container);