Add PlainAndColorizedString.toString.

This prevents unsupported error text from being lost when the
unsupported error text is in the form of a PlainAndColorizedString.

Change-Id: Ibc15a6cd17fea085332a308b58356b8d903a3202
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189901
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes.dart

@@ -166,7 +166,14 @@ class PlainAndColorizedString {
   final String plain;
   final String colorized;
 
+  @override
+  String toString() {
+    assert(false, "Called PlainAndColorizedString.toString: $plain");
+    return 'PlainAndColorizedString:$plain';
+  }
+
   const PlainAndColorizedString(this.plain, this.colorized);
+
   const PlainAndColorizedString.plainOnly(this.plain) : this.colorized = plain;
 }