Add support to dart2js for option --enable-asserts.

tests/language/function_type/function_type10_test.dart

Index: tests/language/function_type/function_type10_test.dart
diff --git a/tests/language/function_type/function_type10_test.dart b/tests/language/function_type/function_type10_test.dart
index a68960a1c93c64db0a347aa7a6fa70193bbd9b78..af95f76e7229cce2d9643f1588dec2ec25e13bfa 100644
--- a/tests/language/function_type/function_type10_test.dart
+++ b/tests/language/function_type/function_type10_test.dart
@@ -19,9 +19,15 @@ import 'package:expect/expect.dart';
 @AssumeDynamic()
 confuse(f) => f;
 
-final bool inCheckedMode =
-    (() { bool result = false; assert(result = true); return result; })();
-
+final bool inCheckedMode = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();

[sra1, 2017/05/19 16:51:35]

Looking at this large duplication I think we should add this logic to
package:expect and remove it from all the tests.
We should be precise with the names and have the name be correct for all
implementations.

final bool typeAssertionsEnabled = ...
final bool assertStatementsEnabled = ...

if we also need inCheckedMode, this would mean `--checked` which is a
combination of the two (on the VM you can control them independently).

final bool inCheckedMode = typeAssertionsEnabled && assertStatementsEnabled;

If you have different names that are a little more future-proof that would be
ok.

[eernst, 2017/05/23 17:09:55]

The duplication is slightly deceptive here because
function_type{0,1,..,99}_test.dart are generated. But it's certainly more
readable to have working definitions of `typeAssertionsEnabled` and
`assertStatementsEnabled` (and I called the third one `checkedModeEnabled`) in a
shared location, and then use that everywhere.

I've added these declarations to pkg/expect/lib/expect.dart and adjusted all
tests touched by this CL to use them.
I think it makes sense to use those names: They are long so they don't easily
clash with other things, and they are used relatively rarely so it is important
that they are highly self-explanatory.

 
 typedef F0<T> = int Function(int y, {int x});
 typedef F1<T> = Function Function(int y, {List<T> x});