Fix bug in SsaConstructionFieldTypes: handle mulitple assignments to the same field.

tests/language/multiple_field_assignment_constructor_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import "compiler_annotations.dart";
+
+var a = [null];
+
+class A {
+  var foo;
+  var bar;
+
+  @DontInline
+  A() {
+    // Currently defeat inlining by using a closure.
+    bar = () => 42;
+    foo = 42;
+    foo = a[0];
+  }
+}
+
+class B {
+  var foo;
+  var bar;
+
+  @DontInline
+  B() {
+    // Currently defeat inlining by using a closure.
+    bar = () => 42;
+    foo = 42;
+    foo = a[0];
+    if (false) foo = 42;
+  }
+}
+
+main() {
+  // Surround the call to [bar] by allocations of [A] and [B] to
+  // ensure their constructors get analyzed first.
+  new A();
+  new B();
+  bar();
+  new A();
+  new B();
+}
+
+@DontInline
+bar() {
+  // Currently defeat inlining by using a closure.
+  Expect.throws(() => new A().foo + 42, (e) => e is NoSuchMethodError);
+  Expect.throws(() => new B().foo + 42, (e) => e is NoSuchMethodError);
+}