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

pkg/compiler/lib/src/resolution/operators.dart

Index: pkg/compiler/lib/src/resolution/operators.dart
diff --git a/pkg/compiler/lib/src/resolution/operators.dart b/pkg/compiler/lib/src/resolution/operators.dart
index b5d83d9eb50eb91132a1492fcf926854a29e92cc..b524cf4b5726ff4fd73c5ae6342ec9e142665dc9 100644
--- a/pkg/compiler/lib/src/resolution/operators.dart
+++ b/pkg/compiler/lib/src/resolution/operators.dart
@@ -53,6 +53,7 @@ class UnaryOperator {
     }
   }
 
+  // ignore: MISSING_RETURN
   static UnaryOperator fromKind(UnaryOperatorKind kind) {
     switch (kind) {
       case UnaryOperatorKind.NOT:
@@ -235,6 +236,7 @@ class BinaryOperator {
     }
   }
 
+  // ignore: MISSING_RETURN
   static BinaryOperator fromKind(BinaryOperatorKind kind) {
     switch (kind) {
       case BinaryOperatorKind.EQ:
@@ -432,6 +434,7 @@ class AssignmentOperator {
     }
   }
 
+  // ignore: MISSING_RETURN
   static AssignmentOperator fromKind(AssignmentOperatorKind kind) {
     switch (kind) {
       case AssignmentOperatorKind.ASSIGN:
@@ -496,6 +499,7 @@ class IncDecOperator {
     }
   }
 
+  // ignore: MISSING_RETURN
   static IncDecOperator fromKind(IncDecOperatorKind kind) {
     switch (kind) {
       case IncDecOperatorKind.INC: