solve syntax error + remedy warnings

Runtime/material/ink_highlight.cs

         }
 
         void _paintHighlight(Canvas canvas, Rect rect, Paint paint) {
-            D.assert(this._shape != null);
             canvas.save();
             if (this._customBorder != null) {
                 canvas.clipPath(this._customBorder.getOuterPath(rect));

Runtime/material/text_theme.cs

         public static TextTheme lerp(TextTheme a, TextTheme b, double t) {
             D.assert(a != null);
             D.assert(b != null);
-            D.assert(t != null);
             return new TextTheme(
                 display4: TextStyle.lerp(a.display4, b.display4, t),
                 display3: TextStyle.lerp(a.display3, b.display3, t),

Runtime/material/theme_data.cs

             IconThemeData iconTheme,
             IconThemeData primaryIconTheme,
             IconThemeData accentIconTheme,
-            MaterialTapTargetSize materialTapTargetSize,
+            MaterialTapTargetSize? materialTapTargetSize,
             ColorScheme colorScheme,
             Typography typography
         ) {

Runtime/painting/text_style.cs

             D.assert(this.fontWeight != null || fontWeightDelta == 0.0);
             D.assert(this.letterSpacing != null || (letterSpacingFactor == 1.0 && letterSpacingDelta == 0.0));
             D.assert(this.wordSpacing != null || (wordSpacingFactor == 1.0 && wordSpacingDelta == 0.0));
-            D.assert(heightFactor != null || (heightFactor == 1.0 && heightDelta == 0.0));
+            D.assert(this.height != null || (heightFactor == 1.0 && heightDelta == 0.0));
 
             string modifiedDebugLabel = "";
             D.assert(() => {
         }
 
         public static TextStyle lerp(TextStyle a, TextStyle b, double t) {
-            D.assert(t != null);
             D.assert(a == null || b == null || a.inherit == b.inherit);
             if (a == null && b == null) {
                 return null;

Runtime/rendering/proxy_box.cs

             Color color = null,
             Color shadowColor = null
         ) : base(child: child,
-            elevation = elevation,
+            elevation : elevation,
             color: color,
             shadowColor: shadowColor ?? new Color(0xFF000000),
             clipper: clipper,

Runtime/service/system_chrome.cs

             );
         }
 
-        public int GetHashCode() {
+        public override int GetHashCode() {
             var hashCode = this.systemNavigationBarColor.GetHashCode();
             hashCode = (hashCode * 397) ^ this.systemNavigationBarDividerColor.GetHashCode();
             hashCode = (hashCode * 397) ^ this.statusBarColor.GetHashCode();