infer null from context, this avoids bottom being introduced

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index f68147180e5529973e12766669e0348cd963a001..9f4aa69f923023697973a431b924f09849a2e168 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -1029,8 +1029,18 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
    */
   @override
   Object visitNullLiteral(NullLiteral node) {
-    // TODO(jmesserly): in strong mode, should we just use the context type?
-    _recordStaticType(node, _typeProvider.bottomType);
+    DartType type;
+    if (_strongMode) {
+      // In strong mode, we use the surrounding context type if we have one.
+      //
+      // Otherwise we use dynamic, because that's what we want to be inferred
+      // by default. For example in `var x = null;` the type of `x` is dynamic.
+      type = InferenceContext.getType(node) ?? _typeProvider.dynamicType;
+    } else {
+      // In Dart 1, null is bottom to make it assignable to everything.
+      type = _typeProvider.bottomType;
+    }
+    _recordStaticType(node, type);
     return null;
   }