Avoid using dart:io we compiling for web

In support of http://go/clientside-dartpad

Change-Id: I379095327e724e4b136986116343dd9e30459d6b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/490560
Auto-Submit: Jonas Jensen <jonasfj@google.com>
Commit-Queue: Jonas Jensen <jonasfj@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

pkg/_fe_analyzer_shared/lib/src/util/colors.dart

@@ -119,6 +119,10 @@ void Function(String) printEnableColorsReason = (_) {};
 /// Note: do not call this method directly, as it is expensive to
 /// compute. Instead, use [enableColors].
 bool _computeEnableColors() {
+  if (const bool.fromEnvironment('dart.library.js_interop')) {
+    printEnableColorsReason("Not enabling colors when compiled for web.");
+    return false;
+  }
   if (!stdout.supportsAnsiEscapes) {
     printEnableColorsReason(
       "Not enabling colors, stdout does not support ANSI colors.",