Fix prefix double underscore with extra underscore

Author: Yui T

src/compiler/utilities.ts

@@ -230,9 +230,19 @@ namespace ts {
         return getSourceTextOfNodeFromSourceFile(getSourceFileOfNode(node), node, includeTrivia);
     }
 
+    function prefixWithUnderscoreUnderscore(identifier: string): boolean {
+        if (identifier.length <= 2) {
+            return false;
+        }
+        else {
+            return identifier.charCodeAt(0) === CharacterCodes._ && identifier.charCodeAt(1) === CharacterCodes._ ? true : false;
+        }
+    }
+    
     // Add an extra underscore to identifiers that start with two underscores to avoid issues with magic names like '__proto__'
+    // TODO(yuisu): comment
     export function escapeIdentifier(identifier: string): string {
-        return identifier.length >= 2 && identifier.charCodeAt(0) === CharacterCodes._ && identifier.charCodeAt(1) === CharacterCodes._ ? "_" + identifier : identifier;
+        return prefixWithUnderscoreUnderscore(identifier)? "_" + identifier : identifier;
     }
 
     // Remove extra underscore from escaped identifier