Add a few more known non-null cases

lib/runtime/dart/convert.js

 dart_library.library('dart/convert', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/async',
   'dart/typed_data',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
 ], function(exports, dart, core, async, typed_data, _internal, collection) {
   'use strict';
@@ skipping 242 matching lines @@
       }
       return core.String.fromCharCodes(bytes, start, end);
     }
     [_convertInvalid](bytes, start, end) {
       let buffer = new core.StringBuffer();
       for (let i = start; dart.notNull(i) < dart.notNull(end); i = dart.notNull(i) + 1) {
         let value = bytes[dartx.get](i);
         if ((dart.notNull(value) & ~dart.notNull(this[_subsetMask])) != 0) value = 65533;
         buffer.writeCharCode(value);
       }
-      return dart.toString(buffer);
+      return buffer.toString();
     }
     bind(stream) {
       return super.bind(stream);
     }
   }
   dart.setSignature(_UnicodeSubsetDecoder, {
     constructors: () => ({_UnicodeSubsetDecoder: [_UnicodeSubsetDecoder, [core.bool, core.int]]}),
     methods: () => ({
       convert: [core.String, [core.List$(core.int)], [core.int, core.int]],
       [_convertInvalid]: [core.String, [core.List$(core.int), core.int, core.int]],
@@ skipping 865 matching lines @@
         }
       }
       if (offset == 0) {
         this.writeString(s);
       } else if (offset < dart.notNull(length)) {
         this.writeStringSlice(s, offset, length);
       }
     }
     [_checkCycle](object) {
       for (let i = 0; i < dart.notNull(this[_seen][dartx.length]); i++) {
-        if (dart.notNull(core.identical(object, this[_seen][dartx.get](i)))) {
+        if (core.identical(object, this[_seen][dartx.get](i))) {
           dart.throw(new JsonCyclicError(object));
         }
       }
       this[_seen][dartx.add](object);
     }
     [_removeSeen](object) {
       dart.assert(!dart.notNull(this[_seen][dartx.isEmpty]));
       dart.assert(core.identical(this[_seen][dartx.last], object));
       this[_seen][dartx.removeLast]();
     }
     writeObject(object) {
       if (dart.notNull(this.writeJsonValue(object))) return;
       this[_checkCycle](object);
       try {
         let customJson = dart.dcall(this[_toEncodable$], object);
         if (!dart.notNull(this.writeJsonValue(customJson))) {
           dart.throw(new JsonUnsupportedObjectError(object));
         }
         this[_removeSeen](object);
       } catch (e) {
         dart.throw(new JsonUnsupportedObjectError(object, {cause: e}));
       }
 
     }
     writeJsonValue(object) {
       if (typeof object == 'number') {
         if (!dart.notNull(object[dartx.isFinite])) return false;
         this.writeNumber(object);
         return true;
-      } else if (dart.notNull(core.identical(object, true))) {
+      } else if (core.identical(object, true)) {
         this.writeString('true');
         return true;
-      } else if (dart.notNull(core.identical(object, false))) {
+      } else if (core.identical(object, false)) {
         this.writeString('false');
         return true;
       } else if (object == null) {
         this.writeString('null');
         return true;
       } else if (typeof object == 'string') {
         this.writeString('"');
         this.writeStringContent(object);
         this.writeString('"');
         return true;
@@ skipping 120 matching lines @@
     })
   });
   class _JsonStringStringifier extends _JsonStringifier {
     _JsonStringStringifier(sink, _toEncodable) {
       this[_sink] = sink;
       super._JsonStringifier(dart.as(_toEncodable, dart.functionType(core.Object, [core.Object])));
     }
     static stringify(object, toEncodable, indent) {
       let output = new core.StringBuffer();
       _JsonStringStringifier.printOn(object, output, toEncodable, indent);
-      return dart.toString(output);
+      return output.toString();
     }
     static printOn(object, output, toEncodable, indent) {
       let stringifier = null;
       if (indent == null) {
         stringifier = new _JsonStringStringifier(output, toEncodable);
       } else {
         stringifier = new _JsonStringStringifierPretty(output, toEncodable, indent);
       }
       dart.dsend(stringifier, 'writeObject', object);
     }
@@ skipping 943 matching lines @@
     convert(codeUnits, start, end) {
       if (start === void 0) start = 0;
       if (end === void 0) end = null;
       let length = codeUnits[dartx.length];
       core.RangeError.checkValidRange(start, end, length);
       if (end == null) end = length;
       let buffer = new core.StringBuffer();
       let decoder = new _Utf8Decoder(buffer, this[_allowMalformed]);
       decoder.convert(codeUnits, start, end);
       decoder.close();
-      return dart.toString(buffer);
+      return buffer.toString();
     }
     startChunkedConversion(sink) {
       let stringSink = null;
       if (dart.is(sink, StringConversionSink)) {
         stringSink = sink;
       } else {
         stringSink = StringConversionSink.from(sink);
       }
       return stringSink.asUtf8Sink(this[_allowMalformed]);
     }
@@ skipping 280 matching lines @@
       if (dart.notNull(this[_isUpgraded])) return this[_upgradedMap][dartx.values];
       return _internal.MappedIterable.new(this[_computeKeys](), dart.fn(each => this.get(each)));
     }
     set(key, value) {
       if (dart.notNull(this[_isUpgraded])) {
         this[_upgradedMap][dartx.set](key, value);
       } else if (dart.notNull(this.containsKey(key))) {
         let processed = this[_processed];
         _JsonMap._setProperty(processed, dart.as(key, core.String), value);
         let original = this[_original];
-        if (!dart.notNull(core.identical(original, processed))) {
+        if (!core.identical(original, processed)) {
           _JsonMap._setProperty(original, dart.as(key, core.String), null);
         }
       } else {
         this[_upgrade]()[dartx.set](key, value);
       }
       return value;
     }
     addAll(other) {
       other[dartx.forEach](dart.fn((key, value) => {
         this.set(key, value);
@@ skipping 38 matching lines @@
       if (dart.notNull(this[_isUpgraded])) return this[_upgradedMap][dartx.forEach](f);
       let keys = this[_computeKeys]();
       for (let i = 0; i < dart.notNull(keys[dartx.length]); i++) {
         let key = keys[dartx.get](i);
         let value = _JsonMap._getProperty(this[_processed], key);
         if (dart.notNull(_JsonMap._isUnprocessed(value))) {
           value = _convertJsonToDartLazy(_JsonMap._getProperty(this[_original], key));
           _JsonMap._setProperty(this[_processed], key, value);
         }
         dart.dcall(f, key, value);
-        if (!dart.notNull(core.identical(keys, this[_data]))) {
+        if (!core.identical(keys, this[_data])) {
           dart.throw(new core.ConcurrentModificationError(this));
         }
       }
     }
     toString() {
       return collection.Maps.mapToString(this);
     }
     get [_isUpgraded]() {
       return this[_processed] == null;
     }
@@ skipping 170 matching lines @@
   exports.LineSplitter = LineSplitter;
   exports.StringConversionSink = StringConversionSink;
   exports.ClosableStringSink = ClosableStringSink;
   exports.UNICODE_REPLACEMENT_CHARACTER_RUNE = UNICODE_REPLACEMENT_CHARACTER_RUNE;
   exports.UNICODE_BOM_CHARACTER_RUNE = UNICODE_BOM_CHARACTER_RUNE;
   exports.Utf8Codec = Utf8Codec;
   exports.UTF8 = UTF8;
   exports.Utf8Encoder = Utf8Encoder;
   exports.Utf8Decoder = Utf8Decoder;
 });