fix PathObserver to avoid try+catch

pkg/observe/lib/src/observable_map.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of observe;
+library observe.src.observable_map;
+
+import 'dart:collection';
+import 'package:observe/observe.dart';
+
 
 // TODO(jmesserly): this needs to be faster. We currently require multiple
 // lookups per key to get the old value.
 // TODO(jmesserly): this doesn't implement the precise interfaces like
 // LinkedHashMap, SplayTreeMap or HashMap. However it can use them for the
 // backing store.
 
 // TODO(jmesserly): should we summarize map changes like we do for list changes?
 class MapChangeRecord<K, V> extends ChangeRecord {
   // TODO(jmesserly): we could store this more compactly if it matters, with
@@ skipping 11 matching lines @@
   /** True if this key was inserted. */
   final bool isInsert;
 
   /** True if this key was removed. */
   final bool isRemove;
 
   MapChangeRecord(this.key, this.oldValue, this.newValue)
       : isInsert = false, isRemove = false;
 
   MapChangeRecord.insert(this.key, this.newValue)
-      : isInsert = true, isRemove = false;
+      : isInsert = true, isRemove = false, oldValue = null;
 
   MapChangeRecord.remove(this.key, this.oldValue)
-      : isInsert = false, isRemove = true;
-
-  /// *Deprecated* compare [key]s instead.
-  @deprecated
-  bool changes(otherKey) => key == otherKey;
+      : isInsert = false, isRemove = true, newValue = null;
 
   String toString() {
     var kind = isInsert ? 'insert' : isRemove ? 'remove' : 'set';
     return '#<MapChangeRecord $kind $key from: $oldValue to: $newValue>';
   }
 }
 
 /**
  * Represents an observable map of model values. If any items are added,
  * removed, or replaced, then observers that are listening to [changes]
@@ skipping 14 matching lines @@
   /**
    * Creates an observable map that contains all key value pairs of [other].
    * It will attempt to use the same backing map type if the other map is a
    * [LinkedHashMap], [SplayTreeMap], or [HashMap]. Otherwise it defaults to
    * [HashMap].
    *
    * Note this will perform a shallow conversion. If you want a deep conversion
    * you should use [toObservable].
    */
   factory ObservableMap.from(Map<K, V> other) {
-    return new ObservableMap<K, V>._createFromType(other)..addAll(other);
+    return new ObservableMap<K, V>.createFromType(other)..addAll(other);
   }
 
-  factory ObservableMap._createFromType(Map<K, V> other) {
+  /** Like [ObservableMap.from], but creates an empty map. */
+  factory ObservableMap.createFromType(Map<K, V> other) {
     ObservableMap result;
     if (other is SplayTreeMap) {
       result = new ObservableMap<K, V>.sorted();
     } else if (other is LinkedHashMap) {
       result = new ObservableMap<K, V>.linked();
     } else {
       result = new ObservableMap<K, V>();
     }
     return result;
   }
@@ skipping 60 matching lines @@
       });
       notifyPropertyChange(#length, len, 0);
     }
     _map.clear();
   }
 
   void forEach(void f(K key, V value)) => _map.forEach(f);
 
   String toString() => Maps.mapToString(this);
 }