diff --git a/flow/BUILD.gn b/flow/BUILD.gn
index 63f1fe529108b..70544c675af91 100644
--- a/flow/BUILD.gn
+++ b/flow/BUILD.gn
@@ -174,6 +174,14 @@ if (enable_unittests) {
       "//third_party/skia",
     ]
 
+    if (!defined(defines)) {
+      defines = []
+    }
+    if (is_win) {
+      # Required for M_PI and others.
+      defines += [ "_USE_MATH_DEFINES" ]
+    }
+
     if (is_fuchsia && flutter_enable_legacy_fuchsia_embedder) {
       sources += [ "layers/fuchsia_layer_unittests.cc" ]
 
diff --git a/flow/matrix_decomposition_unittests.cc b/flow/matrix_decomposition_unittests.cc
index ab0adc172bb31..bf23937415fb4 100644
--- a/flow/matrix_decomposition_unittests.cc
+++ b/flow/matrix_decomposition_unittests.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#define _USE_MATH_DEFINES
-
 #include "flutter/flow/matrix_decomposition.h"
 
 #include <cmath>
diff --git a/lib/ui/BUILD.gn b/lib/ui/BUILD.gn
index 6c72102c2d9b8..841a6afa9c766 100644
--- a/lib/ui/BUILD.gn
+++ b/lib/ui/BUILD.gn
@@ -129,8 +129,15 @@ source_set("ui") {
     "//third_party/skia",
   ]
 
+  if (!defined(defines)) {
+    defines = []
+  }
   if (flutter_enable_skshaper) {
-    defines = [ "FLUTTER_ENABLE_SKSHAPER" ]
+    defines += [ "FLUTTER_ENABLE_SKSHAPER" ]
+  }
+  if (is_win) {
+    # Required for M_PI and others.
+    defines += [ "_USE_MATH_DEFINES" ]
   }
 
   if (is_fuchsia && flutter_enable_legacy_fuchsia_embedder) {
diff --git a/lib/ui/painting/canvas.cc b/lib/ui/painting/canvas.cc
index c066ee3cf772f..68ae4898c176a 100644
--- a/lib/ui/painting/canvas.cc
+++ b/lib/ui/painting/canvas.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#define _USE_MATH_DEFINES
-
 #include "flutter/lib/ui/painting/canvas.h"
 
 #include <cmath>
diff --git a/lib/ui/painting/gradient.cc b/lib/ui/painting/gradient.cc
index e242258e4ddb7..d71e75a495f94 100644
--- a/lib/ui/painting/gradient.cc
+++ b/lib/ui/painting/gradient.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#define _USE_MATH_DEFINES
-
 #include "flutter/lib/ui/painting/gradient.h"
 
 #include "third_party/tonic/converter/dart_converter.h"
diff --git a/lib/ui/painting/path.cc b/lib/ui/painting/path.cc
index b4b7ab55af84b..d0898a4ca283c 100644
--- a/lib/ui/painting/path.cc
+++ b/lib/ui/painting/path.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#define _USE_MATH_DEFINES
-
 #include "flutter/lib/ui/painting/path.h"
 
 #include <cmath>
diff --git a/lib/ui/painting/path_measure.cc b/lib/ui/painting/path_measure.cc
index 16d3de021906f..af9e0e9837e1d 100644
--- a/lib/ui/painting/path_measure.cc
+++ b/lib/ui/painting/path_measure.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#define _USE_MATH_DEFINES
-
 #include "flutter/lib/ui/painting/path_measure.h"
 
 #include <cmath>