Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/io/position_information.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.
 
 /// Source information system mapping that attempts a semantic mapping between
 /// offsets of JavaScript code points to offsets of Dart code points.
 
 library dart2js.source_information.position;
 
 import '../common.dart';
@@ skipping 287 matching lines @@
 }
 
 /// The start, end and closing offsets for a [js.Node].
 class CodePosition {
   final int startPosition;
   final int endPosition;
   final int closingPosition;
 
   CodePosition(this.startPosition, this.endPosition, this.closingPosition);
 
+  // ignore: MISSING_RETURN
   int getPosition(CodePositionKind kind) {
     switch (kind) {
       case CodePositionKind.START:
         return startPosition;
       case CodePositionKind.END:
         return endPosition;
       case CodePositionKind.CLOSING:
         return closingPosition;
     }
   }
@@ skipping 54 matching lines @@
   /// For function expressions the inner position is the closing brace or the
   /// arrow:
   ///
   ///     foo() => () {}
   ///           ^                 // the inner position of the 'foo' function
   ///                  ^          // the inner position of the closure
   ///
   INNER,
 }
 
+// ignore: MISSING_RETURN
 SourceLocation getSourceLocation(SourceInformation sourceInformation,
     [SourcePositionKind sourcePositionKind = SourcePositionKind.START]) {
   if (sourceInformation == null) return null;
   switch (sourcePositionKind) {
     case SourcePositionKind.START:
       return sourceInformation.startPosition;
     case SourcePositionKind.INNER:
       return sourceInformation.closingPosition;
   }
 }
@@ skipping 942 matching lines @@
 
   @override
   CodePosition operator [](js.Node node) {
     CodePosition codePosition = codePositions[node];
     if (codePosition == null) {
       coverage.registerNodesWithoutOffset(node);
     }
     return codePosition;
   }
 }