Disallow negative padding and margins

They're just as crazy in this system as they are in the web.

packages/flutter/lib/painting/box_painter.dart

@@ -45,6 +45,8 @@ class EdgeDims {
   /// The offset from the left
   final double left;
 
+  bool get isNonNegative => top >= 0.0 && right >= 0.0 && bottom >= 0.0 && left >= 0.0;
+
   bool operator ==(other) {
     if (identical(this, other))
       return true;

packages/flutter/lib/src/rendering/shifted_box.dart

@@ -79,6 +79,7 @@ class RenderPadding extends RenderShiftedBox {
   EdgeDims get padding => _padding;
   void set padding (EdgeDims value) {
     assert(value != null);
+    assert(value.isNonNegative);
     if (_padding == value)
       return;
     _padding = value;

packages/flutter/lib/src/widgets/basic.dart

@@ -393,7 +393,10 @@ class Container extends Component {
     this.margin,
     this.padding,
     this.transform
-  }) : super(key: key);
+  }) : super(key: key) {
+    assert(margin == null || margin.isNonNegative);
+    assert(padding == null || padding.isNonNegative);
+  }
 
   final Widget child;
   final BoxConstraints constraints;