dart2js: Implement frequency based naming.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index ddeacb436d9c74023857f1493bfb2e75427b4ca4..d373662d25ba521e62878ba601e8bca781aa9488 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -676,7 +676,9 @@ class JavaScriptBackend extends Backend {
 
   static Namer determineNamer(Compiler compiler) {
     return compiler.enableMinification ?
-        new MinifyNamer(compiler) :
+        compiler.useFrequencyNamer ?
+            new FrequencyBasedNamer(compiler) :
+            new MinifyNamer(compiler) :
         new Namer(compiler);
   }
 
@@ -1462,6 +1464,14 @@ class JavaScriptBackend extends Backend {
    */
   String assembleCode(Element element) {
     assert(invariant(element, element.isDeclaration));
+    var code = generatedCode[element];
+    if (namer is jsAst.TokenFinalizer) {
+      jsAst.TokenCounter counter = new jsAst.TokenCounter();
+      counter.countTokens(code);
+      // Avoid a warning.
+      var finalizer = namer;
+      finalizer.finalizeTokens();
+    }
     return jsAst.prettyPrint(generatedCode[element], compiler).getText();
   }