Clean up the way RenderIntrinsicWidth works (#4340)

The old code had gotten crufty with all the refactors. This simplifies
it down to what it really means.

Also, add a bunch of tests.

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

@@ -530,20 +530,12 @@ class RenderIntrinsicWidth extends RenderProxyBox {
   }
 
   static double _applyStep(double input, double step) {
+    assert(input.isFinite);
     if (step == null)
       return input;
     return (input / step).ceil() * step;
   }
 
-  BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
-    assert(child != null);
-    if (constraints.hasTightWidth)
-      return constraints;
-    final double width = child.getMaxIntrinsicWidth(constraints.maxHeight);
-    assert(width == constraints.constrainWidth(width));
-    return constraints.tighten(width: _applyStep(width, _stepWidth));
-  }
-
   @override
   double getMinIntrinsicWidth(double height) {
     return getMaxIntrinsicWidth(height);
@@ -553,36 +545,46 @@ class RenderIntrinsicWidth extends RenderProxyBox {
   double getMaxIntrinsicWidth(double height) {
     if (child == null)
       return 0.0;
-    double childResult = child.getMaxIntrinsicWidth(height);
-    assert(childResult.isFinite);
-    return _applyStep(childResult, _stepWidth);
+    final double width = child.getMaxIntrinsicWidth(height);
+    return _applyStep(width, _stepWidth);
   }
 
   @override
   double getMinIntrinsicHeight(double width) {
     if (child == null)
       return 0.0;
-    double childResult = child.getMinIntrinsicHeight(_getInnerConstraints(new BoxConstraints.tightForFinite(width: width)).maxWidth);
-    assert(childResult.isFinite);
-    return _applyStep(childResult, _stepHeight);
+    if (!width.isFinite)
+      width = getMaxIntrinsicWidth(double.INFINITY);
+    assert(width.isFinite);
+    final double height = child.getMinIntrinsicHeight(width);
+    return _applyStep(height, _stepHeight);
   }
 
   @override
   double getMaxIntrinsicHeight(double width) {
     if (child == null)
       return 0.0;
-    double childResult = child.getMaxIntrinsicHeight(_getInnerConstraints(new BoxConstraints.tightForFinite(width: width)).maxWidth);
-    assert(childResult.isFinite);
-    return _applyStep(childResult, _stepHeight);
+    if (!width.isFinite)
+      width = getMaxIntrinsicWidth(double.INFINITY);
+    assert(width.isFinite);
+    final double height = child.getMaxIntrinsicHeight(width);
+    return _applyStep(height, _stepHeight);
   }
 
   @override
   void performLayout() {
     if (child != null) {
-      BoxConstraints childConstraints = _getInnerConstraints(constraints);
-      assert(childConstraints.hasTightWidth);
-      if (_stepHeight != null)
-        childConstraints.tighten(height: getMaxIntrinsicHeight(childConstraints.maxWidth));
+      BoxConstraints childConstraints = constraints;
+      if (!childConstraints.hasTightWidth) {
+        final double width = child.getMaxIntrinsicWidth(childConstraints.maxHeight);
+        assert(width.isFinite);
+        childConstraints = childConstraints.tighten(width: _applyStep(width, _stepWidth));
+      }
+      if (_stepHeight != null) {
+        final double height = child.getMaxIntrinsicHeight(childConstraints.maxWidth);
+        assert(height.isFinite);
+        childConstraints = childConstraints.tighten(height: _applyStep(height, _stepHeight));
+      }
       child.layout(childConstraints, parentUsesSize: true);
       size = child.size;
     } else {
@@ -611,27 +613,24 @@ class RenderIntrinsicHeight extends RenderProxyBox {
     RenderBox child
   }) : super(child);
 
-  BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
-    assert(child != null);
-    if (constraints.hasTightHeight)
-      return constraints;
-    final double height = child.getMaxIntrinsicHeight(constraints.maxWidth);
-    assert(height == constraints.constrainHeight(height));
-    return constraints.tighten(height: height);
-  }
-
   @override
   double getMinIntrinsicWidth(double height) {
     if (child == null)
       return 0.0;
-    return child.getMinIntrinsicWidth(_getInnerConstraints(new BoxConstraints.tightForFinite(height: height)).maxHeight);
+    if (!height.isFinite)
+      height = child.getMaxIntrinsicHeight(double.INFINITY);
+    assert(height.isFinite);
+    return child.getMinIntrinsicWidth(height);
   }
 
   @override
   double getMaxIntrinsicWidth(double height) {
     if (child == null)
       return 0.0;
-    return child.getMaxIntrinsicWidth(_getInnerConstraints(new BoxConstraints.tightForFinite(height: height)).maxHeight);
+    if (!height.isFinite)
+      height = child.getMaxIntrinsicHeight(double.INFINITY);
+    assert(height.isFinite);
+    return child.getMaxIntrinsicWidth(height);
   }
 
   @override
@@ -642,7 +641,13 @@ class RenderIntrinsicHeight extends RenderProxyBox {
   @override
   void performLayout() {
     if (child != null) {
-      child.layout(_getInnerConstraints(constraints), parentUsesSize: true);
+      BoxConstraints childConstraints = constraints;
+      if (!childConstraints.hasTightHeight) {
+        final double height = child.getMaxIntrinsicHeight(childConstraints.maxWidth);
+        assert(height.isFinite);
+        childConstraints = childConstraints.tighten(height: height);
+      }
+      child.layout(childConstraints, parentUsesSize: true);
       size = child.size;
     } else {
       performResize();