Use dart.tearoff helper at tearoff sites

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 2a85f77cae73b54bea1a75ceb03ebe27cbfcd755..8be77619d4e130972254cabc9a4d17babfb36048 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -1114,10 +1114,8 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
 
       // For instance members, we add implicit-this.
       // For method tear-offs, we ensure it's a bound method.
-      var code = 'this.#';
-      if (element is MethodElement && !inInvocationContext(node)) {
-        code += '.bind(this)';
-      }
+      var tearOff = element is MethodElement && !inInvocationContext(node);
+      var code = (tearOff) ? 'dart.tearoff(this, #)' : 'this.#';

[Jennifer Messerly, 2015/05/20 16:05:17]

I don't think we should call this "tearoff" in generated code. Inside the Dart
team we've used this name in discussions, but it's not a broader term. I chose
"bind" because that's the name JavaScript uses for this concept, and a search
for "javascript bind" or "function bind" finds the right concept.

[Leaf, 2015/05/20 16:35:17]

So you're saying change dart.bind to both bind the function and add rtti and
then just use that?  That's fine by me.  

[Jennifer Messerly, 2015/05/20 16:50:30]

err, I think so :). What do we still use "dart.bind" for after your CL?

[Leaf, 2015/05/20 17:05:56]

Apparently nothing.  Made the switch.

       return js.call(code, member);
     }
 
@@ -1989,15 +1987,13 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     }
 
     String code;
-    if (member != null && member is MethodElement) {
+    if (member != null && member is MethodElement && !isStatic) {
       // Tear-off methods: explicitly bind it.
+      // TODO(leafp): Attach runtime types to these static tearoffs
       if (_requiresStaticDispatch(target, memberId.name)) {
         return js.call('dart.#.bind(#)', [name, _visit(target)]);
       }
-      if (isStateless(target, target)) {
-        return js.call('#.#.bind(#)', [_visit(target), name, _visit(target)]);

[Jennifer Messerly, 2015/05/20 16:05:17]

this makes me a little sad, as we never generate "normal" JavaScript code
anymore. I guess we can bring it back with the --no-rtti compiler option, at
some point in the future?

[Leaf, 2015/05/20 16:35:17]

Hmm.  I actually kind of felt like it was often an improvement in the generated
code - maybe it's a tradeoff between being idiomatic and being brief?  Can bind
be overridden?  If so then maybe I could work it out so that the original
pattern worked.

[Jennifer Messerly, 2015/05/20 16:50:30]

yeah, exactly ... in JS it's not something you'd typically call a helper for,
because it's so easy to `.bind(this)`. Then again, if we could turn our
dart_runtime.js more into a general "ES6 with types" library, I think that would
make me feel better about having those runtime helpers and using them so much.

But yeah, no worries, this particular one isn't a big deal in the grand scheme.
My concerns are a bit higher level. Maybe we'll get time at some point for a
coherent readability/design review of the JS we're generating.
Kind of. We could alter "Function.prototype.bind". But probably not worth just
for this.

-      }
-      code = 'dart.bind(#, #)';
+      code = 'dart.tearoff(#, #)';
     } else if (_requiresStaticDispatch(target, memberId.name)) {
       return js.call('dart.#(#)', [name, _visit(target)]);
     } else {