Change path-observer to lookup properties aggressively and report errors

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index 298318347762032e9d774b0362607dffdef2d198..cbede1c31543d4143511c52dd307734b2181e3d1 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -150,7 +150,7 @@ void assign(Expression expr, Object value, Scope scope) {
  * searches for the name in local variables, then in global variables,
  * and then finally looks up the name as a property in the model.
  */
-abstract class Scope {
+abstract class Scope implements StringIndexer {
   Scope._();
 
   /** Create a scope containing a [model] and all of [variables]. */
@@ -170,6 +170,10 @@ abstract class Scope {
    */
   Object operator[](String name);
 
+  operator[]=(String name, Object value) {

[Jennifer Messerly, 2014/03/25 22:32:44]

spacing here seems a bit off

[Siggi Cherem (dart-lang), 2014/03/26 00:32:07]

Done.

+    throw new UnimplementedError('[]= is not supported in Scope.');

[Jennifer Messerly, 2014/03/25 22:32:44]

UnsupportedError? in other words, the lack of a setter is intentional right?

[Siggi Cherem (dart-lang), 2014/03/26 00:32:07]

Done.

+  }
+
   /**
    * Returns whether [name] is defined in [model], that is, a lookup
    * would not find a variable with that name, but there is a non-null model