Fix bug in locateSingleElement

tests/compiler/dart2js_extra/locate_single_element_1_test.dart

+// 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.
+
+// Test for locateSingleElement bug.
+
+import 'package:expect/expect.dart';
+
+class T {
+  foo() => 'T.foo';  // This is the single element.
+}
+
+class C implements T {
+  // There is a warning that C does not implement 'foo'.
+}
+
+@NoInline() @AssumeDynamic()
+assumeT(x) {  // returns inferred subtype(T).
+  if (x is T) return x;
+  throw "Not T";
+}
+
+var log = [];
+demo() {
+  log.add(new T());  // T is created.
+  var a = assumeT(new C());  // C is created.
+
+  // The call "a.foo()" should be a NoSuchMethodError, but a bug in
+  // locateSingleElement used to lead to T.foo being inlined.  There is a single
+  // method. T.foo, that matches subtype(T), but it should be rejected because
+  // not all instantiated classes that are subtype(T) have that method.
+  log.add(a.foo());
+}
+
+main() {
+  Expect.throws(demo);
+}