feat(core, testability): PendingAsync service

A new PendingAsync service that is used to track pending async
operations in Angular.  VmTurnZone ties in to this service to track
timers as async tasks and correctly handles canceled timers.  Http
registers async tasks for network requests.  The Testability API uses
the new API to provide a much more useful whenStable implementation.
refactor(DirectiveInjector): better var names and comments
feat(scope): component is the new context

BREAKING CHANGE:

Scope context is set to the component instance that trigged the creation
of the scope (previously it was of a PrototypeMap.)

Repercussions:
1) You can not inject a scope in a component or in the root context any
more.

As the Scope context is set to the Component instance, the scope could
not be injected any more. Components should implements the "ScopeAware"
interface and declare a "scope" setter in order to get a reference to
the scope.

before:

     @Component(...)
     class MyComponent {
       Watch watch;
       Scope scope;

       MyComponent(Dependency myDep, Scope scope) {
          watch = scope.rootScope.watch("expression", (v, p) => ...);
       }
     }

after:

     @Component(...)
     class MyComponent implements ScopeAware {
       Watch watch;

       MyComponent(Dependency myDep) {
         // It is an error to add a Scope argument to the
         // ctor and will result in a DI circular dependency error
         // The scope is never accessible in the class constructor
       }

       void set scope(Scope scope) {
          // This setter gets called to initialize the scope
          watch = scope.watch("expression", (v, p) => ...);
       }
     }

or:

     @Component(...)
     class MyComponent implements ScopeAware {
       Scope scope;

       MyComponent(Dependency myDep) {
         // It is an error to add a Scope argument to the
         // ctor and will result in a DI circular dependency error
         // The scope is never accessible in the class constructor
       }
     }

2) The parent component to an NgForm must have a "$name" field to store
   the form instance.

closes #919
closes #917
chore(travis): revert blacklist Dart SDK 1.7.0-dev.2.0

This reverts commit e2350cac61dc33053504c70be4d1d5411e0f2040.

Current Dart dev sdk version has advanced past the buggy version.

Closes #1447
chore(travis): remove dart version blacklist for 1.7.0-dev.1.0

Closes #1443