Implement string interpolation up to SSA.

dart/frog/leg/tree/unparser.dart

Index: dart/frog/leg/tree/unparser.dart
diff --git a/dart/frog/leg/tree/unparser.dart b/dart/frog/leg/tree/unparser.dart
index b77f6609522006ae61aabe697fb4d636de94cceb..7bcf584290c57dd1154e3a3824e160a874d657c7 100644
--- a/dart/frog/leg/tree/unparser.dart
+++ b/dart/frog/leg/tree/unparser.dart
@@ -229,4 +229,16 @@ class Unparser implements Visitor {
     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);
+  }
 }