Switch from Size.isInfinite to Size.isFinite (#8868)

Fixes #4301

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

@@ -150,7 +150,7 @@ class RenderBlock extends RenderBox
         break;
     }
 
-    assert(!size.isInfinite);
+    assert(size.isFinite);
   }
 
   @override

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

@@ -1533,7 +1533,7 @@ abstract class RenderBox extends RenderObject {
         );
       }
       // verify that the size is not infinite
-      if (_size.isInfinite) {
+      if (!_size.isFinite) {
         final StringBuffer information = new StringBuffer();
         if (!constraints.hasBoundedWidth) {
           RenderBox node = this;
@@ -1658,7 +1658,7 @@ abstract class RenderBox extends RenderObject {
   void performResize() {
     // default behavior for subclasses that have sizedByParent = true
     size = constraints.smallest;
-    assert(!size.isInfinite);
+    assert(size.isFinite);
   }
 
   @override

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

@@ -370,7 +370,7 @@ class RenderStack extends RenderBox
       size = constraints.biggest;
     }
 
-    assert(!size.isInfinite);
+    assert(size.isFinite);
 
     child = firstChild;
     while (child != null) {

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

@@ -108,7 +108,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
       _orientation = configuration.orientation;
     }
     _size = configuration.size;
-    assert(!_size.isInfinite);
+    assert(_size.isFinite);
 
     if (child != null)
       child.layout(new BoxConstraints.tight(_size));