fix(perf): faster looseIdentical implementation

Remove String type check in looseIdentical in JS-mode. It is not necessary as dart2js already compiles `identical` to `===` which compares string contents. Inline call sites.

This improves change detection of plain fields by 40%. On a large internal app the improvement is 5%.

Closes #6364

modules/angular2/src/facade/lang.dart

@@ -235,10 +235,26 @@ class FunctionWrapper {
 
 const _NAN_KEY = const Object();
 
-// Dart can have identical(str1, str2) == false while str1 == str2. Moreover,
+// Dart VM implements `identical` as true reference identity. JavaScript does
-// after compiling with dart2js identical(str1, str2) might return true.
+// not have this. The closest we have in JS is `===`. However, for strings JS
-// (see dartbug.com/22496 for details).
+// would actually compare the contents rather than references. `dart2js`
-bool looseIdentical(a, b) =>
+// compiles `identical` to `===` and therefore there is a discrepancy between
+// Dart VM and `dart2js`. The implementation of `looseIdentical` attempts to
+// bridge the gap between the two while retaining good performance
+// characteristics. In JS we use simple `identical`, which compiles to `===`,
+// and in Dart VM we emulate the semantics of `===` by special-casing strings.
+// Note that the VM check is a compile-time constant. This allows `dart2js` to
+// evaluate the conditional during compilation and inline the entire function.
+//
+// See: dartbug.com/22496, dartbug.com/25270
+const _IS_DART_VM = !identical(1.0, 1);  // a hack
+bool looseIdentical(a, b) => _IS_DART_VM
+  ? _looseIdentical(a, b)
+  : identical(a, b);
+
+// This function is intentionally separated from `looseIdentical` to keep the
+// number of AST nodes low enough for `dart2js` to inline the code.
+bool _looseIdentical(a, b) =>
     a is String && b is String ? a == b : identical(a, b);
 
 // Dart compare map keys by equality and we can have NaN != NaN
@@ -334,4 +350,4 @@ var global = null;
 
 dynamic evalExpression(String sourceUrl, String expr, String declarations, Map<String, String> vars) {
   throw "Dart does not support evaluating expression during runtime!";
-}
+}