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

pkg/compiler/lib/src/resolution/operators.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.
 
 library dart2js.operators;
 
 import '../elements/elements.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector, SelectorKind;
 
@@ skipping 35 matching lines @@
         return NOT;
       case '-':
         return NEGATE;
       case '~':
         return COMPLEMENT;
       default:
         return null;
     }
   }
 
+  // ignore: MISSING_RETURN
   static UnaryOperator fromKind(UnaryOperatorKind kind) {
     switch (kind) {
       case UnaryOperatorKind.NOT:
         return NOT;
       case UnaryOperatorKind.NEGATE:
         return NEGATE;
       case UnaryOperatorKind.COMPLEMENT:
         return COMPLEMENT;
     }
   }
@@ skipping 162 matching lines @@
         return LOGICAL_AND;
       case '||':
         return LOGICAL_OR;
       case '??':
         return IF_NULL;
       default:
         return null;
     }
   }
 
+  // ignore: MISSING_RETURN
   static BinaryOperator fromKind(BinaryOperatorKind kind) {
     switch (kind) {
       case BinaryOperatorKind.EQ:
         return EQ;
       case BinaryOperatorKind.NOT_EQ:
         return NOT_EQ;
       case BinaryOperatorKind.INDEX:
         return INDEX;
       case BinaryOperatorKind.MUL:
         return MUL;
@@ skipping 177 matching lines @@
         return AND;
       case '^=':
         return XOR;
       case '|=':
         return OR;
       default:
         return null;
     }
   }
 
+  // ignore: MISSING_RETURN
   static AssignmentOperator fromKind(AssignmentOperatorKind kind) {
     switch (kind) {
       case AssignmentOperatorKind.ASSIGN:
         return ASSIGN;
       case AssignmentOperatorKind.IF_NULL:
         return IF_NULL;
       case AssignmentOperatorKind.ADD:
         return ADD;
       case AssignmentOperatorKind.SUB:
         return SUB;
@@ skipping 44 matching lines @@
     switch (value) {
       case '++':
         return INC;
       case '--':
         return DEC;
       default:
         return null;
     }
   }
 
+  // ignore: MISSING_RETURN
   static IncDecOperator fromKind(IncDecOperatorKind kind) {
     switch (kind) {
       case IncDecOperatorKind.INC:
         return INC;
       case IncDecOperatorKind.DEC:
         return DEC;
     }
   }
 }