Make string juxtaposition combine properly with string interpolations.

frog/leg/compile_time_constants.dart

Index: frog/leg/compile_time_constants.dart
diff --git a/frog/leg/compile_time_constants.dart b/frog/leg/compile_time_constants.dart
index cefe2f5f2a70f8d0f44862a181f09597555c33cf..f5073682236fc60bc57a2c371f70687a625d5934 100644
--- a/frog/leg/compile_time_constants.dart
+++ b/frog/leg/compile_time_constants.dart
@@ -333,9 +333,9 @@ class MapConstant extends ObjectConstant {
           buffer.add("$isolatePrototype.$name");
         } else {
           value.writeJsCode(buffer, handler);
-        }      
+        }
       }
-      buffer.add("}");      
+      buffer.add("}");
     }
 
     void badFieldCountError() {
@@ -768,6 +768,13 @@ class CompileTimeConstantEvaluator extends AbstractVisitor {
     return new StringConstant(node.dartString);
   }
 
+  Constant visitStringJuxtapostion(StringJuxtaposition node) {
+    if (!node.isInterpolation) {
+      return new StringConstant(node.dartString);
+    }
+    return super.visitStringJuxtapostion(node);
+  }
+
   // TODO(floitsch): provide better error-messages.
   Constant visitSend(Send send) {
     Element element = elements[send];