implement tear offs

lib/runtime/dart_runtime.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var dart, _js_helper;
 (function (dart) {
   'use strict';
 
   let defineProperty = Object.defineProperty;
   let getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 18 matching lines @@
         args.push(name);
       });
     }
     return args;
   }
 
   function dload(obj, field) {
     if (!(field in obj)) {
       throw new core.NoSuchMethodError(obj, field);
     }
-    return obj[field];
+    var result = obj[field];
+    if (typeof result == "function") {
+      // We can't tell if the result needs binding. Fortunately binding the
+      // same function twice has no effect, so we can simply attempt to bind.
+      return result.bind(obj);
+    }
+    return result;
   }
   dart.dload = dload;
 
   // TODO(jmesserly): this should call noSuchMethod, not throw.
   function throwNoSuchMethod(obj, name, args, opt_func) {
     if (obj === void 0) obj = opt_func;
     throw new core.NoSuchMethodError(obj, name, args);
   }
 
   function checkAndCall(f, obj, args, name) {
@@ skipping 41 matching lines @@
     }
     return checkAndCall(obj.get, obj, [index], '[]');
   }
   dart.dindex = dindex;
 
   function dsetindex(obj, index, value) {
     return checkAndCall(obj.set, obj, [index, value], '[]=');
   }
   dart.dsetindex = dsetindex;
 
+  /**
+   * Returns bound `method`.
+   * This helper function avoids needing a temp for `obj`.
+   */
+  function bind(obj, method) {
+    // This is a static bind (dynamic would use `dload`) so no need to check
+    // if `method` is really there on `obj`.`
+    return obj[method].bind(obj);
+  }
+  dart.bind = bind;
+
   function cast(obj, type) {
     // TODO(vsm): handle non-nullable types
     if (obj == null) return obj;
     let actual = getRuntimeType(obj);
     if (isSubtype(actual, type)) return obj;
     throw new _js_helper.CastErrorImplementation(actual, type);
   }
   dart.as = cast;
 
 
@@ skipping 676 matching lines @@
   dart.void = { toString() { return 'void'; } };
   dart.bottom = { toString() { return 'bottom'; } };
 
   dart.JsSymbol = Symbol;
 
   // TODO(jmesserly): hack to bootstrap the SDK
   _js_helper = _js_helper || {};
   _js_helper.checkNum = notNull;
 
 })(dart || (dart = {}));