fix bug with negative margin inside of a justifyContent: center container

src/Layout.js

@@ -362,12 +362,13 @@ var computeLayout = (function() {
     var/*float*/ leadingMainDim = 0;
     var/*float*/ betweenMainDim = 0;
 
-    // If the dimensions of the current node is defined by its children, they
+    var/*float*/ definedMainDim = 0;
-    // are all going to be packed together and we don't need to compute
-    // anything.
     if (!isUndefined(node.layout[dim[mainAxis]])) {
+      definedMainDim = node.layout[dim[mainAxis]];
+    }
+
     // The remaining available space that needs to be allocated
-      var/*float*/ remainingMainDim = node.layout[dim[mainAxis]] -
+    var/*float*/ remainingMainDim = definedMainDim -
       getPaddingAndBorderAxis(node, mainAxis) -
       mainContentDim;
 
@@ -430,7 +431,6 @@ var computeLayout = (function() {
         leadingMainDim = betweenMainDim / 2;
       }
     }
-    }
 
     // <Loop C> Position elements in the main axis and compute dimensions
 

src/__tests__/Layout-test.js

@@ -1055,6 +1055,17 @@ describe('Layout', function() {
     );
   });
 
+  it('should layout negative margin top and center justify content', function() {
+    testLayout(
+      {style: {justifyContent: 'center'}, children: [
+        {style: {marginTop: -6}}
+      ]},
+      {width: 0, height: 0, top: 0, left: 0, children: [
+        {width: 0, height: 0, top: -3, left: 0}
+      ]}
+    );
+  });
+
   xit('should layout flex-wrap', function() {
     testLayout(
       {style: {flexWrap: 'wrap', flexDirection: 'row', width: 100}, children: [