fix #28988, remove throw on Dart1 incompatible is-checks for dartdevc

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
index b2b982a19928b24dbdc69e76e0ae5a357ead6115..14f89cf55ef8da5fe9efde43a77272f1f50dbff0 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/errors.dart
@@ -21,25 +21,6 @@ void ignoreAllErrors(bool flag) {
   JS('', 'dart.__ignoreAllErrors = #', flag);
 }
 
-/// Throw an exception on `is` checks that would return an unsound answer in
-/// non-strong mode Dart.
-///
-/// For example `x is List<int>` where `x = <Object>['hello']`.
-///
-/// These checks behave correctly in strong mode (they return false), however,
-/// they will produce a different answer if run on a platform without strong
-/// mode. As a debugging feature, these checks can be configured to throw, to
-/// avoid seeing different behavior between modes.
-///
-/// (There are many other ways that different `is` behavior can be observed,
-/// however, even with this flag. The most obvious is due to lack of reified
-/// generic type parameters. This affects generic functions and methods, as
-/// well as generic types when the type parameter was inferred. Setting this
-/// flag to `true` will not catch these differences in behavior..)
-void failForWeakModeIsChecks(bool flag) {
-  JS('', 'dart.__failForWeakModeIsChecks = #', flag);

[Leaf, 2017/08/05 02:24:27]

Is it worth keeping this flag, just turned off?  I'm not super keen on keeping
essentially dead code around, but it might be useful to triage DDC bug reports. 
WDYT?

[Jennifer Messerly, 2017/08/07 23:30:53]

yeah ... IMO, less code to keep around as we do all the various migrations &
other Dart 2 impls won't have it, plus if we ever refactor subtype stuff, we
won't need to keep the weak mode impl of subtype around. Also we were trying to
remove "StrongModeError" from DDC's runtime code (there's an issue for that).

but I haven't personally ever used it for triage, so I may be biased. I don't
think it would be too hard to hand edit `dart.is` in the SDK JS to try and solve
a really tricky case (basically `if (result == null) debugger;`). Or we can
revert most of this patch to add back the feature, worst case :)

-}
-
 throwCastError(object, actual, type) {
   var found = typeName(actual);
   var expected = typeName(type);