refactor/simplify nullable inference code

lib/runtime/dart/collection.js

 dart_library.library('dart/collection', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
   'dart/_internal',
   'dart/_js_helper',
   'dart/math'
 ], function(exports, dart, core, _internal, _js_helper, math) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 969 matching lines @@
           return `${leftDelimiter}...${rightDelimiter}`;
         }
         let parts = [];
         IterableBase$()._toStringVisiting[dartx.add](iterable);
         try {
           IterableBase$()._iterablePartsToStrings(iterable, parts);
         } finally {
           dart.assert(core.identical(IterableBase$()._toStringVisiting[dartx.last], iterable));
           IterableBase$()._toStringVisiting[dartx.removeLast]();
         }
-        return dart.toString((() => {
+        return (() => {
           let _ = new core.StringBuffer(leftDelimiter);
           _.writeAll(parts, ", ");
           _.write(rightDelimiter);
           return _;
-        })());
+        })().toString();
       }
       static iterableToFullString(iterable, leftDelimiter, rightDelimiter) {
         if (leftDelimiter === void 0) leftDelimiter = '(';
         if (rightDelimiter === void 0) rightDelimiter = ')';
         if (dart.notNull(IterableBase$()._isToStringVisiting(iterable))) {
           return `${leftDelimiter}...${rightDelimiter}`;
         }
         let buffer = new core.StringBuffer(leftDelimiter);
         IterableBase$()._toStringVisiting[dartx.add](iterable);
         try {
@@ skipping 722 matching lines @@
           }
         }
         if (matchFound) return match;
         dart.throw(_internal.IterableElementError.noElement());
       }
       [dartx.join](separator) {
         if (separator === void 0) separator = "";
         if (this[dartx.length] == 0) return "";
         let buffer = new core.StringBuffer();
         buffer.writeAll(this, separator);
-        return dart.toString(buffer);
+        return buffer.toString();
       }
       [dartx.where](test) {
         dart.as(test, dart.functionType(core.bool, [E]));
         return new (_internal.WhereIterable$(E))(this, test);
       }
       [dartx.map](f) {
         dart.as(f, dart.functionType(dart.dynamic, [E]));
         return new (_internal.MappedListIterable$(E, dart.dynamic))(this, f);
       }
       [dartx.expand](f) {
@@ skipping 4114 matching lines @@
   exports.LinkedHashMapKeyIterable$ = LinkedHashMapKeyIterable$;
   exports.LinkedHashMapKeyIterable = LinkedHashMapKeyIterable;
   exports.LinkedHashMapKeyIterator$ = LinkedHashMapKeyIterator$;
   exports.LinkedHashMapKeyIterator = LinkedHashMapKeyIterator;
   exports.HashSetIterator$ = HashSetIterator$;
   exports.HashSetIterator = HashSetIterator;
   exports.LinkedHashSetCell = LinkedHashSetCell;
   exports.LinkedHashSetIterator$ = LinkedHashSetIterator$;
   exports.LinkedHashSetIterator = LinkedHashSetIterator;
 });