Allow interpolated strings and adjacent strings to be compile-time constants.

frog/value.dart

Index: frog/value.dart
diff --git a/frog/value.dart b/frog/value.dart
index f470ae9afd4ee0c5df0f894b1d8e1b94f93cd011..7268e160ce4a37ce4a530a2bdc285b042b64ae20 100644
--- a/frog/value.dart
+++ b/frog/value.dart
@@ -904,6 +904,11 @@ class EvaluatedValue extends Value implements Hashable {
       span);
   }
 
+  get actualValue() {
+    world.internalError('Should not be getting actual value '
+                        'from raw EvaluatedValue', span);
+  }
+
   bool get needsTemp() => false;
 
   EvaluatedValue get constValue() => this;
@@ -1248,7 +1253,6 @@ class StringValue extends EvaluatedValue {
   }
 }
 
-
 class ListValue extends EvaluatedValue {
   final List<Value> values;