Implement string interpolation up to SSA.

dart/frog/leg/tree/unparser.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 Unparser implements Visitor {
   StringBuffer sb;
   final bool printDebugInfo;
 
   Unparser([this.printDebugInfo = false]);
 
@@ skipping 211 matching lines @@
     visit(node.condition);
     sb.add(' ');
     visit(node.body);
   }
 
   visitParenthesizedExpression(ParenthesizedExpression node) {
     add(node.getBeginToken().value);
     visit(node.expression);
     add(node.getEndToken().value);
   }
+
+  visitStringInterpolation(StringInterpolation node) {
+    visit(node.string);
+    visit(node.parts);
+  }
+
+  visitStringInterpolationPart(StringInterpolationPart node) {
+    sb.add('\${'); // TODO(ahe): Preserve the real tokens.
+    visit(node.expression);
+    sb.add('}');
+    visit(node.string);
+  }
 }