Use messages for (some) public API errors

pkg/front_end/messages.yaml

 # Copyright (c) 2017, 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.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 559 matching lines @@
 Unsupported:
   template: "Unsupported operation: '#name'."
 
 NamedFunctionExpression:
   template: "A function expression can't have a name."
   dart2jsCode: "*ignored*"
 
 ReturnTypeFunctionExpression:
   template: "A function expression can't have a return type."
   dart2jsCode: "*ignored*"
+
+MissingMain:
+  template: "No 'main' method found."
+  tip: "Try adding a method named 'main' to your program."
+  dart2jsCode: MISSING_MAIN
+
+MissingInputFile:
+  template: "Input file not found: #string."
+
+MissingSdkRoot:
+  template: "SDK root directory not found: #string."
+
+MissingSdkSummary:
+  template: "SDK summary not found: #string."
+
+VerificationError:
+  template: "An internal error was found while verifying the output of the compiler. #string"