Implement messages in assert() in dev_compiler.

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
index 351d94d0195a1078c545dbbe257777290b51ca05..336eaa20047ac2e9bea880b14bac082c08a1d96d 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
@@ -851,6 +851,17 @@ class RuntimeError extends Error {
 }
 
 /**
+ * Error thrown when an assert() fails with a message:
+ *
+ *     assert(false, "Message here");
+ */
+class AssertionErrorWithMessage extends AssertionError {

[Jennifer Messerly, 2016/12/12 22:36:02]

do we need a special subtype for this, or was that how dart2js chose to
implement it?

the reason I ask ... it seems a bit overkill to have an entire class just
because of one optional parameter. it is easy to handle this in the toString
method. It's not like AssertionError.toString needs to be optimized with special
subclasses to allow JIT inlining or whatever ... if there's ever a type that
could afford an extra null check, this would be it :)

[Bob Nystrom, 2016/12/13 19:29:51]

This is how dart2js does it, but that's not why I do it here.
If a user passes an explicit null message, I want that to print "Assertion
failed: null". That means I can't rely on checking the message for null to
decide how to display it. I could add a separate bool "_hasMessage" field or
something, but this felt a little cleaner to me.

I can't do the thunk checking here, because the thunk gets forced before we get
here. If evaluating the message parameter throws an exception, I want that to
propagate instead of the AssertionError.

+  final Object _message;
+  AssertionErrorWithMessage(this._message);
+  String toString() => "Assertion failed: ${_message}";
+}
+
+/**
  * Creates a random number with 64 bits of randomness.
  *
  * This will be truncated to the 53 bits available in a double.