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

pkg/compiler/lib/src/io/position_information.dart

Index: pkg/compiler/lib/src/io/position_information.dart
diff --git a/pkg/compiler/lib/src/io/position_information.dart b/pkg/compiler/lib/src/io/position_information.dart
index 35605748134be06cc0aba0d8078a4cb3de855f4e..0400e61535cb427281ccdc37dd81b9043bc5d15f 100644
--- a/pkg/compiler/lib/src/io/position_information.dart
+++ b/pkg/compiler/lib/src/io/position_information.dart
@@ -305,6 +305,7 @@ class CodePosition {
 
   CodePosition(this.startPosition, this.endPosition, this.closingPosition);
 
+  // ignore: MISSING_RETURN
   int getPosition(CodePositionKind kind) {
     switch (kind) {
       case CodePositionKind.START:
@@ -379,6 +380,7 @@ enum SourcePositionKind {
   INNER,
 }
 
+// ignore: MISSING_RETURN
 SourceLocation getSourceLocation(SourceInformation sourceInformation,
     [SourcePositionKind sourcePositionKind = SourcePositionKind.START]) {
   if (sourceInformation == null) return null;