chore(test-bed): add fromInjector constructor to TestBed

Some clients do not generate static factories for test code, and instead
directly depend on TestBed constructor. This commit adds a new
constructor is basically the injector factory method, requiring only
injector vs a full set of parameters.

Once clients have been moved to use fromInjector method (or even better
a transformer run for test code), we can change the constructor
dependencies in a non-breaking way.

Closes #1271

Author: rkirov

lib/mock/test_bed.dart

@@ -20,6 +20,10 @@ class TestBed {
 
   TestBed(this.injector, this.directiveInjector, this.rootScope, this.compiler, this._parser, this.expando);
 
+  TestBed.fromInjector(Injector i) :
+    this(i, i.get(DirectiveInjector), i.get(RootScope), i.get(Compiler),
+        i.get(Parser), i.get(Expando));
+
 
   /**
    * Use to compile HTML and activate its directives.