more inference fixes

Author: kevmoo

example/example.g.dart

@@ -188,7 +188,8 @@ class JsonSerializableGenerator
     // Generate the static factory method
     //
     buffer.writeln();
-    buffer.writeln('$className ${prefix}FromJson(Map json) =>');
+    buffer
+        .writeln('$className ${prefix}FromJson(Map<String, dynamic> json) =>');
     buffer.write('    new $className(');
     buffer.writeAll(
         ctorArguments.map((paramElement) => _jsonMapAccessToField(
@@ -378,15 +379,22 @@ class JsonSerializableGenerator
         return "$varExpression as Map<String, dynamic>";
       }
 
+      // In this case, we're going to create a new Map with matching reified
+      // types.
+
       var itemVal = "v$depth";
       var itemSubVal =
           _writeAccessToJsonValue(itemVal, valueArg, depth: depth + 1);
 
-      // In this case, we're going to create a new Map with matching reified
-      // types.
+      if (itemVal == itemSubVal) {
+        // No mapping of the values is required!
+        return "$varExpression == null ? null :"
+            "new Map<String, $valueArg>.from($varExpression)";
+      }
+
       return "$varExpression == null ? null :"
           "new Map<String, $valueArg>.fromIterables("
-          "($varExpression as Map).keys,"
+          "($varExpression as Map<String, dynamic>).keys,"
           "($varExpression as Map).values.map(($itemVal) => $itemSubVal))";
     } else if (searchType.isDynamic || searchType.isObject) {
       // just return it as-is. We'll hope it's safe.

lib/src/json_serializable_generator.dart

@@ -93,7 +93,8 @@ class JsonHelper extends TypeHelper {
   @override
   String deserialize(DartType targetType, String expression) {
     // TODO: the type could be imported from a library with a prefix!
-    return "new ${targetType.name}.fromJson($expression)";
+    // github.com/dart-lang/json_serializable/issues/19
+    return "new ${targetType.name}.fromJson($expression as Map<String, dynamic>)";
   }
 }
 
@@ -114,5 +115,7 @@ class DateTimeHelper extends TypeHelper {
 
   @override
   String deserialize(DartType targetType, String expression) =>
-      "DateTime.parse($expression)";
+      // TODO(kevmoo) `String` here is ignoring
+      // github.com/dart-lang/json_serializable/issues/19
+      "DateTime.parse($expression as String)";
 }

lib/src/type_helper.dart

@@ -84,7 +84,7 @@ class Person extends Object with _$PersonSerializerMixin {
 
   Person(this.firstName, this.lastName, {this.middleName, this.dateOfBirth});
 
-  factory Person.fromJson(Map json) => _$PersonFromJson(json);
+  factory Person.fromJson(Map<String, dynamic> json) => _$PersonFromJson(json);
 
   bool operator ==(Object other) =>
       other is Person &&
@@ -124,7 +124,7 @@ class Item extends Object with _$ItemSerializerMixin {
 
   Item([this.price]);
 
-  factory Item.fromJson(json) => _$ItemFromJson(json);
+  factory Item.fromJson(Map<String, dynamic> json) => _$ItemFromJson(json);
 
   bool operator ==(Object other) =>
       other is Item &&