Implement string interpolation up to SSA.

dart/frog/leg/typechecker.dart

 // Copyright (c) 2011, 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.
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : types = new Types(), super(compiler);
   String get name() => "Type checker";
   Types types;
 
   void check(Node tree, TreeElements elements) {
@@ skipping 514 matching lines @@
     Type thenType = analyzeNonVoid(node.thenExpression);
     Type elseType = analyzeNonVoid(node.elseExpression);
     if (types.isSubtype(thenType, elseType)) {
       return thenType;
     } else if (types.isSubtype(elseType, thenType)) {
       return elseType;
     } else {
       return objectType;
     }
   }
+
+  visitStringInterpolation(StringInterpolation node) {
+    node.visitChildren(this);
+    return stringType;
+  }
+
+  visitStringInterpolationPart(StringInterpolationPart node) {
+    node.visitChildren(this);
+  }
 }