Move String interpolation constant folding from constant propagator to canonicalizer

tests/language/constant_string_interpolation_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.
+// VMOptions=--optimization-counter-threshold=10 --no-use-osr
+
+import "package:expect/expect.dart";
+
+main() {
+  final a = new A();
+  for (int i = 0; i < 20; i++) {
+    final r = interpolIt(a);
+    Expect.stringEquals("hello home", r);
+  }
+  final b = new B();
+  // Deoptimize "interpolIt".
+  final r = interpolIt(b);
+  Expect.stringEquals("hello world", r);
+}
+
+
+String interpolIt(v) {
+  // String interpolation will be constant folded.
+  return "hello ${v.foo()}";
+}
+
+class A {
+  foo() => "home";
+}
+
+class B {
+  foo() => "world";
+}