Clean up dart:io warnings/errors found by analyzer tests.

tests/standalone/io/file_fuzz_test.dart

Index: tests/standalone/io/file_fuzz_test.dart
diff --git a/tests/standalone/io/file_fuzz_test.dart b/tests/standalone/io/file_fuzz_test.dart
index 81d9d4e289b24f817efba4b2dd98b9d087405766..f90030ed0ecf4f6a359797f7e2d9a61c30eb4b11 100644
--- a/tests/standalone/io/file_fuzz_test.dart
+++ b/tests/standalone/io/file_fuzz_test.dart
@@ -21,15 +21,15 @@ fuzzSyncMethods() {
       doItSync(f.lengthSync);
       doItSync(f.modifiedSync);
       doItSync(f.fullPathSync);
-      doItSync(() => f.openRead().listen(() {}, onError: (e) {}));
+      doItSync(() => f.openRead().listen((_) {}, onError: (e) {}));
       doItSync(f.readAsBytesSync);
       doItSync(f.readAsStringSync);
       doItSync(f.readAsLinesSync);
       typeMapping.forEach((k2, v2) {
         doItSync(() => f.openSync(mode: v2));
         doItSync(() => f.openWrite(mode: v2));
-        doItSync(() => f.readAsStringSync(v2));
-        doItSync(() => f.readAsLinesSync(v2));
+        doItSync(() => f.readAsStringSync(encoding: v2));
+        doItSync(() => f.readAsLinesSync(encoding: v2));
       });
     });
   });
@@ -52,9 +52,9 @@ fuzzAsyncMethods() {
       futures.add(doItAsync(f.readAsLines));
       futures.add(doItAsync(f.readAsString));
       typeMapping.forEach((k2, v2) {
-        futures.add(doItAsync(() => f.open(v2)));
-        futures.add(doItAsync(() => f.readAsString(v2)));
-        futures.add(doItAsync(() => f.readAsLines(v2)));
+        futures.add(doItAsync(() => f.open(mode: v2)));
+        futures.add(doItAsync(() => f.readAsString(encoding: v2)));
+        futures.add(doItAsync(() => f.readAsLines(encoding: v2)));
       });
     });
   });
@@ -76,7 +76,7 @@ fuzzSyncRandomAccessMethods() {
       doItSync(() => opened.writeByteSync(v));
     });
     for (var p in typePermutations(2)) {
-      doItSync(() => opened.writeStringSync(p[0], p[1]));
+      doItSync(() => opened.writeStringSync(p[0], encoding: p[1]));
     }
     for (var p in typePermutations(3)) {
       doItSync(() => opened.readIntoSync(p[0], p[1], p[2]));
@@ -104,7 +104,7 @@ fuzzAsyncRandomAccessMethods() {
         futures.add(doItAsync(() => opened.writeByte(v)));
     });
     for (var p in typePermutations(2)) {
-      futures.add(doItAsync(() => opened.writeString(p[0], p[1])));
+      futures.add(doItAsync(() => opened.writeString(p[0], encoding: p[1])));
     }
     for (var p in typePermutations(3)) {
       futures.add(doItAsync(() => opened.readInto(p[0], p[1], p[2])));