Inserted string validation as separate task in compiler.

frog/leg/compiler.dart

Index: frog/leg/compiler.dart
diff --git a/frog/leg/compiler.dart b/frog/leg/compiler.dart
index a118238e1ecf8e34e915c55349b7211efb24b8f7..1315111903d5af80b645451884439a7ae54ced7c 100644
--- a/frog/leg/compiler.dart
+++ b/frog/leg/compiler.dart
@@ -76,8 +76,8 @@ class Compiler implements Canceler, Logger {
     optimizer = new SsaOptimizerTask(this);
     generator = new SsaCodeGeneratorTask(this);
     emitter = new CodeEmitterTask(this);
-    tasks = [scanner, parser, resolver, checker, builder, optimizer, generator,
-             emitter, compileTimeConstantHandler];
+    tasks = [scanner, parser, resolver, checker, builder,
+             optimizer, generator, emitter, compileTimeConstantHandler];
   }
 
   void ensure(bool condition) {