Add dart_messages package.

pkg/compiler/lib/src/diagnostics/shared_messages.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-// This file is parsed by JavaScript and must not use fancy Dart constructs.
-// It can contain JSON like constructs and "//" comments (but not "/*" "*/").
-// It must have one assignment (`final MESSAGES =`).
-// All strings must be raw strings.
-
-// The messages in this file should meet the following guide lines:
-//
-// 1. The message should be a complete sentence starting with an uppercase
-// letter, and ending with a period.
-//
-// 2. Reserved words and embedded identifiers should be in single quotes, so
-// prefer double quotes for the complete message. For example, "The
-// class '#{className}' can't use 'super'." Notice that the word 'class' in the
-// preceding message is not quoted as it refers to the concept 'class', not the
-// reserved word. On the other hand, 'super' refers to the reserved word. Do
-// not quote 'null' and numeric literals.
-//
-// 3. Do not try to compose messages, as it can make translating them hard.
-//
-// 4. Try to keep the error messages short, but informative.
-//
-// 5. Use simple words and terminology, assume the reader of the message
-// doesn't have an advanced degree in math, and that English is not the
-// reader's native language. Do not assume any formal computer science
-// training. For example, do not use Latin abbreviations (prefer "that is" over
-// "i.e.", and "for example" over "e.g."). Also avoid phrases such as "if and
-// only if" and "iff", that level of precision is unnecessary.
-//
-// 6. Prefer contractions when they are in common use, for example, prefer
-// "can't" over "cannot". Using "cannot", "must not", "shall not", etc. is
-// off-putting to people new to programming.
-//
-// 7. Use common terminology, preferably from the Dart Language
-// Specification. This increases the user's chance of finding a good
-// explanation on the web.
-//
-// 8. Do not try to be cute or funny. It is extremely frustrating to work on a
-// product that crashes with a "tongue-in-cheek" message, especially if you did
-// not want to use this product to begin with.
-//
-// 9. Do not lie, that is, do not write error messages containing phrases like
-// "can't happen".  If the user ever saw this message, it would be a
-// lie. Prefer messages like: "Internal error: This function should not be
-// called when 'x' is null.".
-//
-// 10. Prefer to not use imperative tone. That is, the message should not sound
-// accusing or like it is ordering the user around. The computer should
-// describe the problem, not criticize for violating the specification.
-//
-// Other things to keep in mind:
-//
-// Generally, we want to provide messages that consists of three sentences:
-// 1. what is wrong, 2. why is it wrong, 3. how do I fix it. However, we
-// combine the first two in [template] and the last in [howToFix].
-
-final MESSAGES = {
-};