Work around Platform.ansiSupported missing from checked-in dart binary.

pkg/front_end/lib/src/fasta/colors.dart

Index: pkg/front_end/lib/src/fasta/colors.dart
diff --git a/pkg/front_end/lib/src/fasta/colors.dart b/pkg/front_end/lib/src/fasta/colors.dart
index f3891b917d50387f3ccdc2b196cefd395f55e936..3d4a4323e6b6eb2e8dd02cccf863fe79dc9f9c96 100644
--- a/pkg/front_end/lib/src/fasta/colors.dart
+++ b/pkg/front_end/lib/src/fasta/colors.dart
@@ -98,7 +98,13 @@ String white(String string) => wrap(string, WHITE_COLOR);
 /// Note: do not call this method directly, as it is expensive to
 /// compute. Instead, use [CompilerContext.enableColors].
 bool computeEnableColors(CompilerContext context) {
-  if (!Platform.ansiSupported) {
+  bool ansiSupported;

[karlklose, 2017/03/16 13:01:47]

Initialize with 'false'?

[ahe, 2017/03/16 13:11:09]

I have to distinguish between three cases:

* We know that ansi mode isn't supported.

* We know that ansi mode is supported.

* We don't know if ansi mode is supported.

Null represents the latter, and this is used to avoid returning in the case on
line 107.

+  try {
+    ansiSupported = Platform.ansiSupported;
+  } catch (e) {

[karlklose, 2017/03/16 13:01:47]

Could you be more specific about the exception type that you expect when this
getter is not available?

[ahe, 2017/03/16 13:11:10]

Done.

+    // Ignored.

[karlklose, 2017/03/16 13:01:46]

Maybe add something like: 'the getter is not supported in this version of the
VM'.

[ahe, 2017/03/16 13:11:09]

Done.

+  }
+  if (ansiSupported == false) {
     if (context.options.verbose) {
       print("Not enabling colors, 'Platform.ansiSupported' is false.");
     }
@@ -119,7 +125,7 @@ bool computeEnableColors(CompilerContext context) {
     return false;
   }
 
-  if (Platform.isWindows) {
+  if (ansiSupported == true && Platform.isWindows) {
     if (context.options.verbose) {
       print("Enabling colors as OS is Windows.");
     }