Add explicit types to for-in loops (#839)

Work around for dart-lang/sdk#31884 in the
dev.17.0 version of the SDK

Author: natebosch

build_runner/lib/src/generate/watch_impl.dart

@@ -306,7 +306,7 @@ class WatchImpl implements BuildState {
 
 Map<AssetId, ChangeType> _collectChanges(List<List<AssetChange>> changes) {
   var changeMap = <AssetId, ChangeType>{};
-  for (var change in changes.expand((l) => l)) {
+  for (AssetChange change in changes.expand((l) => l)) {
     var originalChangeType = changeMap[change.id];
     if (originalChangeType != null) {
       switch (originalChangeType) {

build_runner/test/common/test_phases.dart

@@ -144,7 +144,7 @@ void checkBuild(BuildResult result,
 
   final unhiddenOutputs = <String, dynamic>{};
   final unhiddenAssets = new Set<AssetId>();
-  for (final id in outputs?.keys ?? const []) {
+  for (final String id in outputs?.keys ?? const []) {
     if (id.startsWith(r'$$')) {
       final unhidden = id.substring(2);
       unhiddenAssets.add(makeAssetId(unhidden));

build_runner/test/generate/build_definition_test.dart

@@ -214,7 +214,7 @@ main() {
             environment.reader);
         var generatedACopyId = makeAssetId('a|lib/a.txt.copy');
         var generatedACloneId = makeAssetId('a|lib/a.txt.clone');
-        for (var id in [generatedACopyId, generatedACloneId]) {
+        for (AssetId id in [generatedACopyId, generatedACloneId]) {
           var node = originalAssetGraph.get(id) as GeneratedAssetNode;
           node.wasOutput = true;
           node.needsUpdate = false;