Implement type inference of constructor invocations in Fasta.

pkg/front_end/testcases/inference/conflicts_can_happen.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 class I1 {
   int x;
 }
@@ skipping 11 matching lines @@
 }
 
 class C1 implements A, B {
   /*error:INVALID_METHOD_OVERRIDE*/ get a => null;
 }
 
 // Still ambiguous
 class C2 implements B, A {
   /*error:INVALID_METHOD_OVERRIDE*/ get a => null;
 }
+
+main() {}