Make compile-time errors catchable

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index c11ea2e1a97da25e0c73490a216d7ec025492dc6..c9d85cafb7fd72787398c38658a5805ef1541e76 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -571,3 +571,11 @@ class CyclicInitializationError extends Error {
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }
+
+
+class SyntaxError extends Error {

[Lasse Reichstein Nielsen, 2016/09/15 05:41:35]

I have two objections here:

1. This will represent compile-time errors, not all of which are syntax errors.
A better name would be `CompileTimeError`.

2. I'm not sure I want to expose this class. The dart2js code won't need it for
anything, and its current implementation isn't adding anything important for the
user - it's just wrapping a message.
I hope no user will want to catch a CompileTimeError directly. If you catch all
errors, it makes sense to catch CompileTimeError too, but singling out
CompileTimeErrors with an `on CompileTimeError catch (e)` is probably just being
too clever.
Making it a VM-private class that still extends Error is sufficient for me, and
I'd like to see if it's sufficient for other users as well.

The subclasses of Error that we have are there because they carry important
information for debugging and for creating a good error string in a consistent
way, and not really so you can identify them by class (not that people don't but
it's not the intent). This class does neither: It doesn't expose important
information as data, and it isn't intended for users to create instances of to
help them have consistent error messages, so it has no reason to be user
visible.

[hausner, 2016/09/15 21:28:28]

Renamed SyntaxError to _CompileTimeError. The class is VM-only and not
user-visible.

+  final String errorMsg;
+  SyntaxError(this.errorMsg);
+  String toString() => errorMsg;
+}
+