Throw NoSuchMethod or IllegalArgument on bad operators or bad arguments to

tests/language/src/StringConcatTest.dart

Index: tests/language/src/StringConcatTest.dart
diff --git a/tests/language/src/StringConcatTest.dart b/tests/language/src/StringConcatTest.dart
index fbc303df440a2f1a138469b359521d453f94a5e9..b25b1002593f7c5d485e29e72552b68af484064b 100644
--- a/tests/language/src/StringConcatTest.dart
+++ b/tests/language/src/StringConcatTest.dart
@@ -45,6 +45,12 @@ class StringConcatTest {
     } catch (TypeError e) {
       // In type checked mode.
       nofExceptions++;
+    } catch (IllegalArgumentException e) {
+      // TODO(floitsch): IllegalArgumentException might not be correct. If
+      // number operations are supposed to be implemented as double-dispatch
+      // then we should get a NoSuchMethodException instead. In frog the
+      // argument is eagerly checked and throws an IllegalArgumentException.
+      nofExceptions++;
     }
 
     // Check that compile time constants are canonicalized.