Fix multi span text ruler cache lookup failure. (#12023)

lib/web_ui/lib/src/engine/text/ruler.dart

@@ -759,8 +759,13 @@ class ParagraphRuler {
 
   MeasurementResult cacheLookup(
       EngineParagraph paragraph, ui.ParagraphConstraints constraints) {
+    final String plainText = paragraph._plainText;
+    if (plainText == null) {
+      // Multi span paragraph, do not use cache item.
+      return null;
+    }
     final List<MeasurementResult> constraintCache =
-        _measurementCache[paragraph._plainText];
+    _measurementCache[plainText];
     if (constraintCache == null) {
       return null;
     }

lib/web_ui/test/paragraph_test.dart

@@ -62,4 +62,38 @@ void main() async {
       );
     }
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/37744
+  test('measures heights of multiple multi-span paragraphs', () {
+    const double fontSize = 20.0;
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: fontSize,
+    ));
+    builder.addText('1234567890 1234567890 1234567890 1234567890 1234567890');
+    builder.addText('1234567890 1234567890 1234567890 1234567890 1234567890');
+    builder.pushStyle(TextStyle(fontWeight: FontWeight.bold));
+    builder.addText('span0');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(ParagraphConstraints(width: fontSize * 5.0));
+    expect(
+        paragraph.height, closeTo(fontSize * 3.0, 0.001)); // because it wraps
+
+    // Now create another builder with just a single line of text so
+    // it tries to reuse ruler cache but misses.
+    final ParagraphBuilder builder2 = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: fontSize,
+    ));
+    builder2.addText('span1');
+    builder2.pushStyle(TextStyle(fontWeight: FontWeight.bold));
+    builder2.addText('span2');
+    final Paragraph paragraph2 = builder2.build();
+    paragraph2.layout(ParagraphConstraints(width: fontSize * 5.0));
+    expect(paragraph2.height, closeTo(fontSize, 0.001)); // because it wraps
+  });
 }